One of the well-known environmental challenges nowadays is the handling of used tires. There are estimations that every year, at least one billion tires are discarded around the world. These scrap tires are an ecological predicament in all countries in which automobiles and trucks are a standard mode of transportation. Over the years, many more tires cast off in monumental piles than recycled or burned. It is estimated that in the US alone there are in excess of 1 billion tires in illegal tire piles, generating dangerous conditions of uncontrollable fires, air pollution as well as health hazards.
To date, most discarded tires are burned, assisting in alleviating an unending energy crisis. However, since the recognition by meteorologists of pending earth-warming trends, burning tires is quickly becoming an unacceptable solution, and in some countries even an illegal one. Furthermore, to date, many of the waste tires are simply shredded and buried in landfills. This solution has also become an undesirable one as more and more countries recognize the danger in underground buried tires or tire parts, due to the adverse effect on diminishing underground supplies of fresh water. In addition, tire piles serve as breeding grounds to colonies of disease infected rodents and incubation hot beds for dangerous and deadly insects. It is therefore becomes clear that recycling must be the only acceptable and sustainable solution to the increasing problem of scrap tires. However, in order to ensure that recycling is indeed the acceptable solution to the problem, the quality of the product received in a recycling process must be such that it will be suitable for the various uses of these products.
Ground Tire Rubber (hereinafter: “GTR”) powder may be prepared by at least two different processes, and the surface morphology of the rubber particles is expected to depend on the method of their production. The size, shape and morphological features of the particles' surface, can affect the properties of the end products, the polymer/rubber composites.
Ambient ground rubber is obtained by shredding and grinding (milling) the tire rubber at or above ambient temperature. This process produces a sponge-like surface of the granulated rubber crumbs which have considerably greater surface area for a given particle size, than do cryogenically ground rubber particles.
Cryogenically ground rubber is obtained by grinding (turbo milling) the tire rubber at below the brittle temperature (glass transition temperature) of the rubber. However, the cryogenic process may produce undesirable particle morphology (structure) and might generally give a lower elastic recovery when compared with the ambient ground rubber.
Vulcanized rubber may be retrieved from various rubber materials. For example, from rubber products such as rubber tires, weather strips, hoses, piston cups, belts, floor carpets and the like, or any other used rubber materials that are to be disposed, non-used materials such as scraps of virgin rubber materials and unsuccessfully molded rubber articles. Although various types of rubber materials may be used in a mixture, still, it is usually preferred to use a single type of material in order to obtain a reclaimed article having stable properties.
GTR crumb is a cluster of hydrocarbon main chains with mono-, di-, polysulfidic cross-links having developed surface and configuration. The average molecular weight of the rubber can be maintained at a high level of over 100,000 and typically over 150,000. In some cases, an average molecular weight of over 200,000 may be maintained.
The major drawback of using conventional crumbs is that the cross-link bonds created by the vulcanization process are relatively very strong. This fact results in rubber compound molecules having a “memory” effect, which enables rubber items to regain their original shape after being subjected to a deforming force. For example, a rubber band that has undergone stretching will return to its original shape. Therefore, even when GTR is processed into crumbs, the rubber molecules retain much of their physical properties. The strength of the cross-link bonds makes binding of the crumb rubber within a virgin rubber matrix rather difficult, whereas the bonding between the new rubber and the GTR is weak.
GTR powders are typically inert materials and are non-compatible with virgin elastomer matrix materials. Thus, combining GTR with various polymers results in compositions that exhibit poor properties due to the weak interfacial adhesion existing between the crosslinked GTR particles and the polymer. Consequently, GTR powders are of limited use as filler in rubber or plastic compositions, as high levels of GTR generally lower the rheology and overall physical properties of rubber compositions. As such, less than 4% by weight of GTR may commonly be mixed when manufacturing new tires, as when higher percentage of GTR is mixed while manufacturing the compositions, the new tires' properties such as strain modulus begin to degrade.
Reclaimed GTR is a crumbed rubber which was treated with aggressive chemicals that attack the carbon backbone as well as other molecular bonds. The resultant rubber material is typically reused (recycled) in small proportions as processing aids (“reclaimed rubber”) or as diluents with fresh rubber compounds. However, the presence of reclaimed rubber in such mixtures, adversely affects the physical and dynamic properties of the final vulcanizate. Rubber which was reclaimed by using such approaches tends to demonstrate poor physical properties.
Reclaimed GTR is a crumb rubber that was treated either mechanically or under combination of mechanical and chemical processing, in order to break most of the sulfur cross-links as well as some of the carbon chains. The problem associated with this type of technology is the non-homogeneous de-vulcanization, which is caused by poor diffusion of the chemical agent into the rubber particles.
Unfortunately, various reclaiming processes have certain drawbacks that limit their use on a large scale, as some of the processes involve relatively high temperatures. The use of high temperatures combined with mass transfer limited reactions, lead to a greater opportunity for thermal degradation reactions to occur.
Furthermore, the slow reaction rate also leads to thermal cracking of the polymer, which in turn reduces the molar mass of the reclaimed polymer, thereby degrading the mechanical properties, while producing light hydrocarbon gaseous products. Thermally degrading reactions might shorten the length of rubber polymer chains or otherwise change their chemical structures such that their mechanical properties are adversely affected, thereby limiting their potential use in new rubber products.
Certain current methods use high temperatures (e.g., 150-250° C.) to digest the elastomeric material being recycled, and require stirring of the mixture for many hours (e.g., 5 to 12 hours). This results in processes that are characterized as high energy consumers and the end result is a degraded material that is being reclaimed. For example, the intense heat and mechanical shear actions of a thermo-mechanical process might cause some polymeric chain backbone to break down. In addition, because this process does not use chemicals, it is not possible to selectively cleave targeted polysulfidic bonds. Some processes, such as solvent extraction, utilize large quantities of chemicals or solvents that might damage the environment, or otherwise involve follow up complicated solvent or chemical recovery and treatment processes. Still, other reclamation processes, such as microwave or ultrasonic processing, employ relatively complicated equipment that is difficult to obtain and difficult to operate efficiently on an industrial production scale.
Rubber that has been reclaimed by using such approaches tends to have poor physical properties. For example, conventionally reclaimed rubber may have tensile strength of about 3.5 to 5 MPa, elongation of 200-250% while natural rubber may have tensile strength over 20 MPa and elongation of above 400%.
Therefore, there is a need for a rubber composition that comprises GTR and a method for manufacturing such a product, that do not compromise desirable properties of the rubber composition.